1. Field of the Invention
The present invention relates to a solid-state imaging device, a method of driving a solid-state imaging device, and an imaging apparatus.
2. Description of the Related Art
Solid-state imaging devices are divided into two main types, that is, a charge-transfer-type solid-state imaging device, such as a CCD (charge coupled device) image sensor, and a MOS solid-state imaging device, such as a CMOS (complementary metal oxide semiconductor) image sensor. In recent years, the MOS solid-state imaging device, for example, the CMOS image sensor has been used for a camera module having low power consumption that is provided in a mobile apparatus, such as a cellular phone, or a high-sensitivity electronic still camera.
For example, in the electronic still camera required for high resolution, since a still picture obtained by pressing the shutter of the camera needs to have high resolution, a solid-state imaging device having a larger number of pixels has been used. However, in the electronic still camera, a user can see a subject through an electronic view finder or a small monitor. Therefore, it is necessary to read image signals having relatively low resolution at high speed in a stage in which the subject is viewed.
In the CMOS image sensor of the related art having color filters for R (red), G (green), and B (blue) color cording, in order to read the image signals having relatively low resolution at high speed, pixel information on n pixels in the horizontal direction (n is an integer that is equal to or larger than 2) and n pixels in the vertical direction is added, and is then read as pixel information on one pixel (for example, JP-A-2004-266369). Hereinafter, a driving mode in which the pixel information on the n pixels in the horizontal direction and the n pixels in the vertical direction is added and is then read as the pixel information on one pixel is referred to as a horizontal/vertical n/n read mode.
FIG. 10 is a block diagram illustrating the basic structure of a CMOS image sensor according to the related art. As shown in FIG. 10, a CMOS image sensor 100 has a pixel array section 102, a constant current source unit 103, a column signal processing unit (column processing unit) 104, a vertical scanning circuit 105, and a horizontal scanning circuit 106, horizontal signal lines 107, an output processing unit 108, and a timing generator 109 that are provided on a semiconductor substrate 101.
The pixel array unit 102 includes a plurality of pixels (not shown) arranged in a two-dimensional matrix, and each of the plurality of pixels has a photoelectric conversion element. A vertical signal line (not shown) is arranged for each row of pixels.
The CMOS image sensor 100 having the above-mentioned structure performs a shutter operation for removing charge stored in the photoelectric conversion element and a read operation for read the charge of an electric signal that is obtained by the photoelectric conversion of the photoelectric conversion element and is stored in the photoelectric conversion element. The two operations will be described below.
FIG. 11 is a timing chart illustrating a driving mode for read all pixels. FIG. 11 shows the read operation and the shutter operation for an n-th row of pixels and subsequent rows of pixels represented in the unit of one frame. Here, a pixel subjected to the shutter operation is represented by a character ‘s’, and a pixel subjected to the read operation is represented by a character ‘r’.
In FIG. 11, T11 and T12 correspond to storage times in the n-th row of pixels. FIG. 11 shows the driving mode for reading all the pixels that is continuously performed. In this case, the storage times T11 in all rows of pixels have the same interval, and the storage times T12 in all rows of pixels have the same interval. When the setting of the storage time is not changed, the storage times T11 and T12 are equal to each other.
FIG. 12 is a timing chart illustrating a horizontal/vertical 2/2 read mode. FIG. 12 shows the read operation and the shutter operation for an n-th row of pixels and subsequent rows of pixels represented in the unit of one frame. Here, similar to FIG. 11, a pixel subjected to the shutter operation is represented by a character ‘s’, and a pixel subjected to the read operation is represented by a character ‘r’. Periods from the shutter operation to the read operation are represented by storage times T21 and T22.
As shown in FIG. 12, in the horizontal/vertical 2/2 read mode, the pixels are driven as follows: an n-th row of pixels and an (n+2)-th row of pixels are simultaneously processed in the vertical direction, and then an (n+1)-th row of pixels and an (n+3)-th row of pixels are simultaneously processed.
FIG. 12 shows the horizontal/vertical 2/2 read mode that is continuously performed. In this case, the storage times T21 in all rows of pixels have the same interval, and the storage times T22 in all rows of pixels have the same interval. When the setting of the storage time is not changed, the storage times T21 and T22 are equal to each other.
Next, pixel scanning methods during the shutter operation and the read operation will be described with reference to FIGS. 13A and 13B. FIG. 13A shows a general RGB pixel arrangement (Bayer pattern) in the CMOS image sensor, and FIG. 13B shows the scanning direction.
As shown in FIG. 13A, in the Bayer pattern, odd-numbered rows of pixels indicate GR rows, and even-numbered rows of pixels indicate GB rows. When the shutter operation or the read operation is performed on the Bayer pattern, scanning is performed on the pixels in the order represented by arrow A, as shown in FIG. 13B. In FIG. 13B, HD indicates a horizontal synchronization signal.
FIGS. 14A to 14C are timing charts illustrating the difference among pixel driving timings due to the difference among the pixel driving modes according to the related art.
FIG. 14A is a timing chart illustrating an all-pixel read mode, FIG. 14B is a timing chart illustrating a horizontal/vertical 2/2 read mode, and FIG. 14C is a timing chart illustrating a horizontal/vertical 3/3 read mode. In FIGS. 14A to 14C, rectangular signals Gr and Gb indicate each row of pixels to be scanned as shown in FIG. 13B. More specifically, the signal Gr indicates a place in which the GR row of pixels is scanned, and the signal Gb indicates a place in which the GB row of pixels is scanned.
As shown in FIG. 13A, the GR row indicates an odd-numbered row of pixels, and the GB row indicates an even-numbered row of pixels. Therefore, the odd-numbered row of pixel is substituted for the signal Gr, and the even-numbered row of pixels is substituted for the signal Gb. Then, scanning is performed on the signals Gr and Gb.
In the all-pixel read mode, as shown in FIG. 14A, the GR rows and the GB rows are sequentially read. However, in the horizontal/vertical 2/2 read mode, since two pixels are added in the vertical direction, two GR rows, that is, an n-th row and an (n+2)-th row are simultaneously scanned, and then two GB rows, that is, an (n+1)-th row and an (n+3)-th row are simultaneously scanned, as shown in FIG. 14B.
In the horizontal/vertical 3/3 read mode, since three pixels are added in the vertical direction, three GR rows, that is, an n-th row, an (n+2)-th row, and an (n+4)-th row are simultaneously scanned, and then three GB rows, that is, an (n+3)-th row, an (n+5)-th row, and an (n+7)-th row are simultaneously scanned, as shown in FIG. 14C.
As shown in FIGS. 14A to 14C, different pixel driving modes cause the numbers of horizontal synchronization signals HD11, HD12, and HD13 generated for the n-th to (n+11)-th rows to be different from one another, and thus the time required to process all rows of pixels depends on the pixel driving mode. As a result, as shown in FIGS. 15A to 15C, different pixel driving modes cause tilt angles θ11, θ12, and θ13 at which rows of pixels are scanned to be different from one another.